1. Field of Invention
The invention relates to a smectic, liquid-crystal mixture based on rod-shaped molecules having only one side chain and certain phenylpyrimidine derivatives.
2. Description of the Related Art
Due to their unusual combination of anisotropic and fluid behavior, liquid crystals (LCs) have found a multiplicity of possible applications in electro-optical switching and display devices.
In addition to the nematic liquid-crystal phases which have been employed for some time, smectic liquid-crystal phases, in particular ferroelectric liquid crystal phases (FLCs), have increasingly also been used recently.
Switching and display devices which contain ferroelectric liquid-crystal mixtures ("FLC light valves") have been disclosed, for example, in EP-B 0 032 362 (=U.S. Pat. No. 4,367,924). LC light valves are devices which, for example due to electrical switching, change their optical transmission characteristics in such a way that incident (and possibly re-reflected) light is modulated in intensity. Examples are the knownwatch and calculator displays or LC displays in the area of office communication and television. However, these also include light shutters, as employed, for example, in photocopiers and printers. Spatial light modulators are also areas of application of LC light valves (see Liquid Crystal Device Handbook, Nikkan Kogyo Shimbun, Tokyo, 1989; ISBN 4-526-02590-9C 3054, and the papers cited therein).
The, electro-optical switching and display devices mentioned are generally constructed in such a way that the FLC layer is surrounded on both sides by layers which are usually, in this sequence starting from the FLC layer, at least one alignment layer, electrodes and a limiting plate (for example made of glass). In addition, they contain at least one polarizer if they are operated in "guest-host" mode or in reflective mode, or two polarizers if the transmissive birefringence mode is used. The switching and display elements may, if desired, contain further auxiliary layers, such as diffusion barrier layers or insulation layers.
The abovementioned alignment layers are usually rubbed films of organic polymers or obliquely vapor-deposited silicon oxide and vary from one display manufacturer to the next.
At a sufficiently small separation of the limiting plates, the alignment layers bring the FLC molecules into a configuration in which the molecules lie with their long axes parallel to one another and the smectic planes are arranged perpendicular or inclined to the alignment layer. In this arrangement, the molecules have two equivalent alignments, between which they can be switched by applying an electric field in a pulsed manner.
In order to achieve a uniform planar alignment in the S.sub.C phase over the entire display, it is advantageous for the phase sequence of the liquid-crystal mixture to be as follows, with decreasing temperature: isotropic-nematic -smectic A-smectic C (see, for example, K. Flatischler et al., Mol. Cryst. Liq. Cryst. 131, 21 (1985); T. Matsumoto et al., pp. 468-470, Proc. of the 6th Int. Display Research Conf., Japan Display, 30 Sep.-2 Oct. 1986, Tokyo, Japan; M. Murakami et al., ibid., pp. 344-347).
In addition, for ferroelectric (chiral, smectic) liquid-crystal mixtures, the condition must be satisfied that the pitch of the helix is sufficiently large in the S.sub.C phase to prevent the formation of a helix in the display and is so large in the N.sup.* phase that the cooling process in the display is not accompanied by formation of a twisted state, but instead by formation of a homogeneous, nematic phase. The formation of a uniform, planar alignment in the display is necessary, inter alia, to achieve high contrast.
The to- and- fro switching of the molecules (and thus bright or dark setting for a fixed polarizer setting) takes place, as mentioned above, due to pulsed application of an electric field. Owing to the histability of the FLC molecules, voltage must only be present for an alignment change. A subdivision of the display into individual pixels is achieved by the known matrix arrangement of the electrodes. The electrodes are generally on the insides of the outer plates of the display, with the rows on one outer plate and the columns on the other. In the crossover areas, the pixels B, the liquid crystal between lines and columns is switched. A basic description of multiplex addressing for FLC displays is given, for example, in Proc. SID 28/2, 211 (1978), and Ferroelectrics 94, 3 (1989).
The response time .tau. of the FLC mixture in the display is inversely proportional to the spontaneous polarization P.sub.s and is in the region of microseconds. ##EQU1## E=strength of the applied electric field .eta.=rotational viscosity
.theta.=tilt angle or half the switching angle PA1 a) at least one bisester of the formula I ##STR1## in which R.sup.1 and R.sup.2 are identical or different and are unbranched or branched alkyl chains having 1 or 3 to 20 carbon atoms, in which, in addition, one or more H atoms may be replaced by fluorine, and PA1 b) at least one compound having only one side chain of the formula II EQU R.sup.1 (--A.sup.1) (--M.sup.1) (--A.sup.2) (--M.sup.2).sub.d (--A.sup.3).sub.e (--M.sup.3).sub.f (--A.sup.4)--H II PA1 R.sup.1 is a straight-chain or branched alkyl radical having 1 to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--; --CO--O--, --O--CO--, --O--CO--O-- or --Si (CH.sub.3).sub.2 --; PA1 A.sup.1, A.sup.2, A.sup.3 and A.sup.4 are identical or different and are 1,4-phenylene, in which one or two H atoms may be replaced by F or CN, pyridine-2,5-diyl, in which one or two H atoms may be replaced by F, pyrimidine-2,5-diyl, in which one or two H atoms may be replaced by F, trans-1,4-cyclohexylene, 1,3,4-thiadiazole-2,5-diyl or naphthalene-2,6-diyl; PA1 M.sup.1, M.sup.2 and M.sup.3 are identical or different and are --CO--O--, --O--CO--, --CH.sub.2 --O--, --O--CH.sub.2 -- or --CH.sub.2 --CH.sub.2 --, and PA1 a, b, c, d, e and f are zero or one, with the proviso that the sum a+c+e is 0, 1, 2 or 3. PA1 4-(5-heptanoyloxypyrimidin-2-yl)phenyl heptanoate, PA1 4-(5-octanoyloxypyrimidin-2-yl)phenyl heptanoate, PA1 4-(5-nonanoyloxypyrimidin-2-yl)phenyl heptanoate, PA1 4-(5-decanoyloxypyrimidin-2-yl)phenyl heptanoate, PA1 4-(5-undecanoyloxypyrimidin-2-yl)phenyl heptanoate, PA1 4-(5-dodecanoyloxypyrimidin-2-yl)phenyl heptanoate, PA1 4-(5-tridecanoyloxypyrimidin-2-yl)phenyl heptanoate, PA1 4-(5-heptanoyloxypyrimidin-2-yl)phenyl octanoate, PA1 4-(5-octanoyloxypyrimidin-2-yl)phenyl octanoate, PA1 4-(5-nonanoyloxypyrimidin-2-yl)phenyl octanoate, PA1 4-(5-decanoyloxypyrimidin-2-yl)phenyl octanoate, PA1 4-(5-undecanoyloxypyrimidin-2-yl)phenyl octanoate, PA1 4-(5-dodecanoyloxypyrimidin-2-yl)phenyl octanoate, PA1 4-(5-tridecanoyloxypyrimidin-2-yl)phenyl octanoate, PA1 4-(5-heptanoyloxypyrimidin-2-yl)phenyl nonanoate, PA1 4-(5-octanoyloxypyrimidin-2-yl)phenyl nonanoate, PA1 4-(5-nonanoyloxypyrimidin-2-yl)phenyl nonanoate, PA1 4-(5-decanoyloxypyrimtdin-2-yl)phenyl nonanoate, PA1 4-(5-undecanoyloxypyrimidin-2-yl)phenyl nonanoate, PA1 4-(5-dodecanoyloxypyrimidin-2-yl)phenyl nonanoate, PA1 4-(5-tridecanoyloxypyrimidin-2-yl)phenyl nonanoate, PA1 4-(5-heptanoyloxypyrimidin-2-yl)phenyl decanoate, PA1 4-(5-octanoyloxypyrimidin-2-yl)phenyl decanoate, PA1 4-(5-nonanoyloxypyrimidin-2-yl)phenyl decanoate, PA1 4-(5-decanoyloxypyrimidin-2-yl)phenyl decanoate, PA1 4-(5-undecanoyloxypyrimidin-2-yl)phenyl decanoate, PA1 4-(5-dodecanoyloxypyrimidin-2-yl)phenyl decanoate, PA1 4-(5-tridecanoyloxypyrimidin-2-yl)phenyl decanoate, PA1 4-(5-heptanoyloxypyrimidin-2-yl)phenyl undecanoate, PA1 4-(5-octanoyloxypyrimidin-2-yl)phenyl undecanoate, PA1 4-(5-nonanoyloxypyrimidin-2-yl)phenyl undecanoate, PA1 4-(5-decanoyloxypyrimidin-2-yl)phenyl undecanoate, PA1 4-(5-undecanoyloxypyrimidin-2-yl)phenyl undecanoate, PA1 4-(5-dodecanoyloxypyrimidin-2-yl)phenyl undecanoate, PA1 4-(5-tridecanoyloxypyrimidin-2-yl)phenyl undecanoate, PA1 4-(5-heptanoyloxypyrimidin-2-yl)phenyl dodecanoate, PA1 4-(5-octanoyloxypyrimidin-2-yl)phenyl dodecanoate, PA1 4-(5-nonanoyloxypyrimidin-2-yl)phenyl dodecanoate, PA1 4-(5-decanoyloxypyrimidin-2-yl) phenyl dodecanoate, PA1 4-(5-undecanoyloxypyrimidin-2-yl)phenyl dodecanoate, PA1 4-(5-dodecanoyloxypyrimidin-2-yl)phenyl dodecanoate, PA1 4-(5-tridecanoyloxypyrimidin-2-yl)phenyl dodecanoate, PA1 4-(5-heptanoyloxypyrimidin-2-yl)phenyl tridecanoate, PA1 4-(5-octanoyloxypyrimidin-2-yl)phenyl tridecanoate, PA1 4-(5-nonanoyloxypyrimidin-2-yl)phenyl tridecanoate, PA1 4-(5-decanoyloxypyrimidin-2-yl)phenyl tridecanoate, PA1 4-(5-undecanoyloxypyrimidin-2-yl)phenyl tridecanoate, PA1 4-(5-dodecanoyloxypyrimidin-2-yl)phenyl tridecanoate, PA1 4-(5-tridecanoyloxypyrimidin-2-yl)phenyl tridecanoate. PA1 a) 1.60 g of nonanoic acid are dissolved in 50 ml of absolute dichloromethane, and 2.7 g of 4-(5-benzyl-oxypyrimidin- 2-yl) phenol are added. 2.1 g of dicyclohexylcarbodiimide and 0.05 g of dimethylaminopyridine are added, and the mixture is stirred at room temperature for 18 hours. The mixture is filtered, and the solvent is stripped off in vacuo. The residue is chromatographed on silica gel. The product, 4-(5-benzyloxypyrimidin-2-yl)phenyl nonanoate, is recrystallized from n-hexane. PA1 Yield: 3.5 g PA1 b) 3.5 g of 4-(5-benzyloxypyrimidin-2-yl)pheny1 nonanoate are dissolved in 100 ml of THF, 0.5 g of 10% Pd on activated charcoal is added, and the mixture is hydrogenated at 25.degree. C. with stirring. The mixture is filtered through corolite, and the solvent is removed in vacuo, giving 2.5 g of 4-(5-hydroxyprimidin-2-yl)phenyl nonanoate. PA1 R.sup.1 is a straight-chain alkyl radical having 1 to 14 carbon atoms, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--CO--, --CO--O--, --O--CO--, --O--CO--O-- or --Si(CH.sub.3).sub.2 --; PA1 A.sup.1, A.sup.2, A.sup.3 and A.sup.4 are identical or different and are 1,4-phenylene, in which one or two H atoms may also be replaced by F, pyrimidine-2,5-diyl, in which one or two H atoms may also be replaced by F, trans-1,4-cyclohexylene, 1,3,4-thiadiazole-2,5-diyl or pyridine-2,5-diyl, in which one or two H atoms may also be replaced by F. PA1 derivatives of phenylpyrimidine, as described, for example, in WO 86/06401 and U.S. Pat. No. 4,874,542, PA1 meta-substituted aromatic compounds having a six-membered ring, as described, for example, in EP-A 0 578 054, PA1 silicon compounds, as described, for example, in EP-A 0 355 008, PA1 hydroquinone derivatives, as described, for example, in German Patent Application P 4 243 705, PA1 pyridylpyrimidines, as described, for example, in WO 92/12974, PA1 phenylbenzoates, as described, for example, in P. Keller, Ferroelectrics 58 (1984), 3, and J. W. Goodby et al., Liquid Crystals and Ordered Fluids, Vol. 4, New York, 1984, and PA1 macrocyclic compounds, as described, for example, in EP-A 0 528 415. PA1 optically active phenylbenzoates, as described, for example, in P. Keller, Ferroelectrics 58 (1984), 3, and J. W. Goodby et al., Liquid Crystals and Ordered Fluids, Vol. 4, New York, 1984, PA1 optically active oxirane ethers, as described, for example, in EP-A 0 263 437 and WO-A 93/13093 , PA1 optically active oxirane esters, as described, for example, in EP-A 0 292 954, PA1 optically active dioxolane ethers, as described, for example, in EP-A 0 351 746, PA1 optically active dioxolane esters, as described, for example, in EP-A 0 361 272, and PA1 optically active tetrahydrofuran-2-carboxylic esters, as described, for example, in EP-A 0 355 561. PA1 A. Phenylpyrimidines of the formula III ##STR3## in which: R.sub.1 and R.sub.2 are identical or different and are a straight-chain or branched alkyl group having 1 to 18 or 3 to 18 carbon atoms respectively, where a CH.sub.2 group adjacent to the oxygen in one of the radicals R.sup.1 and R.sup.2 may also be replaced by --CO--; ##STR4## and IV ##STR5## in which: R.sub.1 and R.sub.2 are identical or different and are a branched or unbranched alkyl group having 1 to 18 or 3 to 18 carbon atoms respectively, where a CH.sub.2 group adjacent to the oxygen may also be replaced by --CO--; ##STR6## B. Metasubstituted aromatic compounds having a six-membered ring, of the formula V ##STR7## in which R.sup.1 and R.sup.2 are identical or different, straight-chain or branched alkyl radicals having 1 to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups my also be replaced by --O--, --CO--, --CO--O--, --O--CO--, --O--CO--O-- or --Si(CH.sub.3).sub.2 --; PA1 A.sup.1, A.sup.2 and A.sup.3 are identical or different and are 1,4-phenylene, in which one or two H atoms may be replaced by F, pyridine-2,5-diyl, in which one or two H atoms may be replaced by F, pyrimidine-2,5-diyl, in which one or two H atoms may be replaced by F, trans-1,4-cyclohexylene, in which one or two H atoms may be replaced by --CN and/or --CH.sub.3, or 1,3,4-thiadiazole-2,5-diyl; PA1 X.sup.1, X.sup.2, X.sup.3, X.sup.4, X.sup.5, X.sup.6, X.sup.7 and X.sup.8 are CH or N, where the number of N atoms per six-membered ring is 0, 1 or 2, and PA1 a, b, c, d, e and f are zero or one, with the proviso that the sum a+c+e is 0, 1 or 2. PA1 C. Carbonates of the formula VI EQU R.sup.1 --O--CO--O--(--A.sup.1).sub.a (--M.sup.1).sub.b (--A.sup.2).sub.c (--M.sup.2).sub.d (--A.sup.3).sub.e (--O).sub.f --R.sup.2 VI PA1 R.sup.1 and R.sup.2 are identical or different and are a straight-chain or branched alkyl group having 1 to 22 or 3 to 22 carbon atoms respectively; PA1 A.sup.1, A.sup.2 and A.sup.3 are identical or different and are 1,4-phenylene, in which one or two H atoms may also be replaced by F, pyrimidine-2,5-diyl, in which one or two H atoms may also be replaced by F, or pyridine-2,5-diyl, in which one or two H atoms may also be replaced by F; PA1 M.sup.1 and M.sup.2 are identical or different and are --O--, --CO--, --CO--O--, --O--CO--, --CH.sub.2 --O--, --O--CH.sub.2 -- or --CH.sub.2 --CH.sub.2 ; PA1 a, b, c, d, e and f are zero or one, with the proviso that the sum a+c+e is 1, 2 or 3. PA1 D. Silicon compounds of the formula VII EQU R.sup.1 (--A.sup.1).sub.i (--M.sup.1).sub.k (--A.sub.2).sub.1 (--M.sub.2).sub.m (--A.sup.3).sub.n --R.sup.2 VII PA1 R.sup.1 is a straight-chain or branched alkyl group having 1 to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CO--O--, --O--CO-- or --O--CO--O--; PA1 R.sup.2 is straight-chain or branched alkyl having 1 to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CO--O--, --O--CO-- or --O--CO--O--, with the proviso that one CH.sub.2 group not bonded to oxygen has been replaced by --Si(CH.sub.3).sub.2 --; PA1 A.sup.1, A.sup.2 and A.sup.3 are identical or different and are 1,4-phenylene, in which one or two H atoms may be replaced by F, trans-1,4-cyclohexylene, pyridine-2,5-diyl, in which one or two H atoms may be replaced by F, pyrimidine-2,5-diyl, in which one or two H atoms may each be replaced by F, or 1,3,4-thiadiazole-2,5-diyl; PA1 M.sup.1 and M.sup.2 are identical or different and are --CO--O--, --O--CO--, --CH.sub.2 --O-- or --O--OH.sub.2 --, and i, k, l, m and n are zero or 1, with the proviso that i+1+n=2 or 3. PA1 E. Hydroquinone derivatives of the formula VIII ##STR9## in which R.sup.1 and R.sup.2 are identical or different, straight-chain or branched alkyl radicals having i or 3 to 16, preferably 1 or 3 to 10 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--, --O--CO--, --CO--O-- or --O--CO--O--, PA1 R.sup.3 is --CH.sub.3, --CF.sub.3 or --C.sub.2 H.sub.5, PA1 A.sup.1 and A.sup.2, independently of one another, are ##STR10## F. Pyridylpyrimidines of the formula (IX) ##STR11## in which A is N and B is CH or A is CH and B is N, C is N and PA1 D is CH or C is CH and D is N, where one or two CH groups may be replaced by CF groups, and PA1 R.sup.1 and R.sup.2 are identical or different, straight-chain or branched alkyl radicals having I to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CO--O--, --O--CO-- or --O--CO--O--. PA1 G. Phenylbenzoates of the formula X ##STR12## in which R.sup.1 and R.sup.2 are identical or different, straight-chain or branched alkyl radicals having 1 to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CO--O--, --O--CO-- or --O--CO--O--; PA1 M.sup.1 and M.sup.2 are identical or different and are --CO--O-- or --O--CO--, and PA1 a, b, c, d and e are zero or one, with the proviso that a+c+e=2 or 3 and b+d=1 or 2. PA1 H. Optically active phenylbenzoates of the formula XI ##STR13## in which R.sup.1 and R.sup.2 are identical or different, straight-chain or branched alkyl radicals having 1 to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CO--O--, --O--CO-- or --O--CO--O--, and in which at least one of the radicals R.sup.1 and R.sup.2 is chiral and non-racemic; PA1 M.sup.1 and M.sup.2 are identical or different and are --CO--O--, --O--CO-- or a single bond, and PA1 a, b, c, d and e are zero or one, with the proviso that a+c+e is 2 or 3. PA1 I. Optically active oxirane ethers of the formula XII ##STR14## in which the symbols and indices have the following meanings: * is a chiral center; PA1 R.sup.1 is a straight-chain or branched alkyl radical having 1 to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CO--O--, --O--CO--, --O--CO--O-- or --Si(CH.sub.3).sub.2 --, or the following optically active group: ##STR15## R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6 and R.sup.7 , independently of one another, are H or a straight-chain or branched alkyl radical having 1 to 16 or 3 to 16 carbon atoms respectively; PA1 P is --CH.sub.2 -- or --CO--; PA1 A.sup.1, A.sup.2 and A.sup.3 are identical or different and are 1,4-phenylene, in which one or two H atoms may be replaced by F, pyridine-2,5-diyl, in which one or two H atoms may each be replaced by F, pyrimidine-2,5-diyl, in which one or two H atoms may be replaced by F, trans-1,4-cyclohexylene, in which one or two H atoms may be replaced by --CN and/or --CH.sub.3, or 1,3,4-thiadiazole-2,5-diyl; M.sup.1 and M.sup.2 are identical or different and are --CO--O--, --O--CO--, --CH.sub.2 --O--, --O--CH.sub.2 -- or --CH.sub.2 --CH.sub.2 --, and a, b, c, d and e are zero or 1. PA1 J. Optically active oxirane esters of the formula XIII ##STR16## where the symbols and indices have the following meanings: * is a chiral center; PA1 R.sup.1 is a straight-chain or branched alkyl radical having 1 to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CO--O--, --O--CO--, --O--CO--O--or --Si(CH.sub.3).sub.2 --; PA1 R.sup.2, R.sup.3 and R.sup.4 are identical or different and are H or a straight-chain or branched alkyl radical having 1 to 16 carbon atoms; PA1 A.sup.1, A.sub.2 and A.sup.3 are identical or different and are 1,4-phenylene, in which one or two H atoms may be replaced by F, pyridine-2,5-diyl, in which one or two H atoms may be replaced by F, pyrimidine-2,5-diyl, in which one or two H atoms may be replaced by F, trans-1,4-cyclohexylene, in which one or two H atoms may be replaced by --CN and/or --CH.sub.3, or 1,3,4-thiadiazole-2,5-diyl; PA1 M.sup.1 and M.sup.2 are identical or different and are --CO--O--, --O--CO--, --CH.sub.2 --O--, --O--CH.sub.2 -- or --CH.sub.2 --CH.sub.2 --; PA1 a, b, c, d and e are zero or one. PA1 K. Optically active dioxolane ethers of the formula XIV ##STR17## where the symbols and indices have the following meanings: * is a chiral center; PA1 R.sup.1 is a straight-chain or branched alkyl radical having 1 to 22 or 3 to 22 carbon atoms respectively, where one or two non-adjacent --CH.sub.2 -- groups may also be replaced by --O--, --CO--, --CO--O--, --O--CO--, --O--CO--O-- or --Si(CH.sub.3).sub.2 --; PA1 R.sup.2, R.sup.3 and R.sup.4 are identical or different and are H or a straight-chain or branched alkyl radical having 1 to 16 carbon atoms, where R.sup.2 and R.sup.3 together may alternatively be --(CH.sub.2).sub.5 --; PA1 A.sup.1, A.sup.2 and A.sup.3 are identical or different and are 1,4-phenylene, in which one or two H atoms may be replaced by F, pyridine-2,5-diyl, in which one or two H atoms may be replaced by F, pyrimidine-2,5-diyl, in which one or two H atoms may be replaced by F, trans-1,4-cyclohexylene, in which one or two H atoms may be replaced by --CN and/or --CH.sub.3, or 1,3,4-thiadiazole-2,5-diyl; PA1 M.sup.1 and M.sup.2 are identical or different and are --CO--O--, --O--CO--, --CH.sub.2 --O--, --O--CH.sub.2 -- or --CH.sub.2 --CH.sub.2 --, and a, b, c, d and e are zero or one. PA1 L. Optically active dioxolane esters of the formula XV ##STR18## in which R.sup.1 is a straight-chain or branched alkyl radical having 1 to 16 carbon atoms where one or more non-adjacent --CH.sub.2 - groups may also be replaced by --O--, --CO--, --O--CO-- or --CO--O--; PA1 R.sup.2, R.sup.3 and R.sup.4 are identical or different and are H or a straight-chain alkyl radical having 1 to 16 carbon atoms; PA1 A.sup.1, A.sup.2 and A.sup.3 are identical or different and are 1,4-phenylene, in which one or two H atoms may be replaced by F, pyridine-2,5-diyl, in which one or two H atoms may be replaced by F, pyrimidine-2,5-diyl, in which one or two H atoms may be replaced by F, trans-1,4-cyclohexylene, in which one or two H atoms may be replaced by --CN and/or --CH.sub.3, or 1,3,4-thiadiazole-2,5-diyl; PA1 M.sup.1 and M.sup.2 are identical or different and are --CO--O--, --O--CO--, --CH.sub.2 --O--, --O--CH.sub.2 -- or --CH.sub.2 --CH.sub.2 --, and a, b, c, d and e are zero or one. PA1 M. Macrocyclic compounds of the formula XVI ##STR19## where n is 0 or 1, and PA1 Y is --CO-- (t-butyl) or --CO-- (adamantyl). PA1 C.sub.y is a mediocyclic or macrocyclic carbon ring having 8 or more ring members, where this ring may also contain fused benzene rings and --O--, --N--, --S--, --Si-- and --B-- as hetero atoms; PA1 S.sub.p is an alkyl group having 1 to 20 carbon atoms in which one or more non-adjacent --CH.sub.2 -- groups may be replaced by --O--, --S--, --CO--, --O--CO--, --NH--CO--, --O--COO--, --NH--CO--NH--, --NH--CO--O--, --SO.sub.2 --, --Si (CH.sub.3).sub.2 --, --CH.dbd.CH-- or --CmC--; PA1 A.sub.n is SiX.sup.1 X.sup.2 X.sup.3, where PA1 X.sup.1 is a single bond and PA1 X.sup.2 and X.sup.3, independently of one another, are a single bond, an alkyl group or an alkoxy group;
In addition to the spontaneous polarization, the tilt angle .theta., i.e. the angle between the n-director, i.e. the average molecule direction, and the layer normals, is of considerable importance. Together with the birefringence .DELTA.n and the layer thickness d, it affects the brightness of the display in accordance with the relationship: ##EQU2## where T.sub.o is the intensity and .lambda. the wavelength of the incident light.
In the case of a matrix arrangement of the electrodes in the display, the columns are usually the electrodes to which information-carrying pulses (also known as column or data pulses) are applied. The lines are then activated sequentially in a stroboscope-like manner by electric pulses, which is the prerequisite for information transfer to the pixels of the lines. An important property of the display is the time necessary for building up or changing an image. For many applications, it should be as short as possible.
Since the lines are addressed sequentially, a crucial factor is the time for which a line has to be addressed to enter information. This writing time is shorter, the shorter the voltage pulses necessary for switching the liquid crystal. In general, the maximum voltage which has to be applied is prescribed by the choice of driver, so that the pulse width necessary for switching should be kept as small as possible.
To good approximation, the product of the requisite pulse width and voltage (=pulse height) is constant, i.e. independent of the voltage, so that the pulse area just necessary for switching (CPA=critical pulse area) represents a parameter which characterizes the speed of the liquid crystal well. The CPA should be as small as possible.
The, rotational viscosity can thus also be determined as follows (see above formula for .tau.): ##EQU3##
It is furthermore advantageous for the LC mixture in the display to have a high margin and low flicker (J. Dijon et al., Ferroelectrics 113 (1991) 371).
Margin is taken to mean the voltage range, for a given addressing scheme, in which the pulse height must be so that the LC phase switches fully. The margin is affected by the bias, i.e. the ratio of line and data pulse voltage. The margin should be as large as possible in order to compensate for thickness and temperature variations in the display.
In the case of multiplex addressing, the molecules of the unselected lines experience a deflection from their rest state due to the data pulses and then relax again. The variation in brightness caused by this is known as flicker. The flicker results in a reduction in contrast. Rieker et al. [Phys. Rev. Lett. 59, 2658 (1987)] have shown that, on cooling from the isotropic phase through the S.sub.A phase, the S.sub.C phase in displays forms a so-called chevron geometry, i.e. the layers are bent. For this reason, the effective tilt angle .theta..sub.eff must be employed in the above relationship between the tilt angle and transmission. The effective tilt angle is the angle between the projections of smectic normals and the optical axis of the liquid crystal onto the glass surface of the cell.
A display can either be operated in the chevron geometry which forms naturally during the cooling operation or in the so-called quasi-bookshelf geometry (QBG), into which the liquid crystal can be brought by specific field treatment (see, for example, H. Rieger etal., SID 91 Digest (Anaheim) 1991, p. 396).
It is almost impossible to achieve good values for the majority of the abovementioned parameters using individual substances. For this reason, mixtures of various substances have been used for some time (see, for example, L. A. Beresnev, L. M. Blinov, Zh. Vsws. Khim. 0-VA28 (1983) 149). Such mixtures generally comprise an achiral base mixture and optically active dopes.
The schiral base mixture should ensure a broad S.sub.C phase in a favorable temperature range. Furthermore, the achiral base mixture should have the phase sequence I-N-S.sub.A -S.sub.C and the lowest possible melting point. The optically active dopes then generally serve to induce ferroelectricity in the mixture, for pitch compensation and for matching of the optical and dielectric anisotropies.
In spite of the successes achieved by the mixtures hitherto in the provision of novel LC materials, the development of FLC mixtures, or components for such mixtures, can in no way be regarded as complete. The manufacturers of display elements ("displays") continue to be interested in a broad range of different mixtures, and thus also individual components, for diverse areas of application.
It is known that certain derivatives of phenylpyrimidine, in particular 5-alkyl-2-(4-alkoxyphenyl)pyrimidines, can form S.sub.C, S.sub.A and N phases (D. Demus and H. Zaschke, "Flussigkristalle in Tabellen" [Liquid Crystals in Tables], VEB Deutscher Verlag far Grundstoffindustrie, Leipzig 1974, pp. 260-261) and in addition can be converted into FLC mixtures by addition of optically active dopes (see, for example, M. L. Blinov et al., Ferroelectrics 59 (1984) 1-201 EP-A 0 206 228 and EP-A 0 225 195).
It is furthermore known that it is advantageous to employ at least three different phenylpyrimidine derivatives in ferroelectric liquid-crystal mixtures (WO 86/06401). EP-A 0 356 672 describes bisesters of 2-(4-hydroxy-phenyl)-5-hydroxypyrimidine in combination with certain optically active phenylpyrimidines as components of FLC mixtures.